1. Field of the Invention
The present invention relates generally to the field of electronic components for optical and broadband communication systems, and more particularly to techniques implement in integrated circuit (IC) technology for cascading broadband amplifiers while preserving their broadband frequency response.
2. Background
Amplifiers used in optical and broadband communication systems typically need to amplify low frequency signals. This requirement makes cascading two or more broadband amplifiers difficult and expensive since a large capacitor is required to block the direct current (dc) bias of the output of one amplifier stage from the input of the subsequent amplifier stage. This blocking capacitor must be large enough (e.g., high capacitance) to pass the full frequency range of the cascaded amplifiers, which can extend down to dc.
Unfortunately, a large capacitor presents various problems that make cascading broadband amplifiers difficult and expensive to implement in certain IC technologies, such as Monolithic Microwave Integrated Circuit (MMIC) technology. To address these problems, several conventional techniques for implementing cascaded broadband amplifiers using IC technology have been proposed.
A first technique uses a large off-chip capacitor in between the amplifier stages. The large physical size of a capacitor capable of passing the required frequency range has significant parasitic effects that make it difficult to design properly.
A second technique uses a small coupling capacitor between the high frequency amplifier stages to pass the high frequencies, and an off-chip parallel path low frequency amplifier to provide dc restoration. The use of a parallel path amplifier requires near perfect match of phase and amplitude through the crossover network, which is difficult to achieve in practice.
A third technique uses a small coupling capacitor and an off-chip high gain feedback amplifier to raise the input impedance of the subsequent amplifier stage at low frequencies. With this technique it is difficult to achieve the high resistor-capacitor (RC) constant required for low frequencies (i.e., KHz range).
Accordingly, a solution is needed for providing adjustable, dc level-shifting capability in a distributed amplifier without using a blocking capacitor and without sacrificing the broadband frequency response of the cascaded amplifiers.
The present invention overcomes the deficiencies of conventional techniques by providing an integrated circuit comprising cascaded amplifiers integrated with an adjustable, distributed level-shifting network. The cascaded amplifiers are direct-coupled together without the use of a dc blocking capacitor. The distributed level-shifting network is coupled to one or more amplifiers in the cascade and provide the desired dc levels. The level-shifting network is adjustable through, for example, one or more variable resistors to balance the crossover frequency response of the cascaded amplifiers and/or to compensate for process variations.
In one embodiment of the present invention, the distributed level-shifting network includes a resistor-capacitor (RC) network coupled in series with a variable resistor and offset voltage supply for balancing the crossover frequency response of the cascaded amplifiers. The variable resistor can be an active device (e.g., transistor) configured to function as a variable resistor. The variable resistor can be included in each amplifier or a single variable resistor can be shared by all of the amplifiers in the cascade.
In another embodiment of the present invention, the distributed level-shifting network is coupled in shunt with the input of an amplifier and includes a level-shifting capacitor coupled in shunt with at least one diode in series with an optional coupling resistor. The level-shifting network can be coupled in series with a variable resistor and offset voltage supply for balancing the crossover frequency response of the amplifier. One or more diodes can also be added for temperature compensation. The variable resistor can be an active device (e.g., transistor) configured to function as a variable resistor. The variable resistor can be included in each amplifier or a single variable resistor can be shared by all of the amplifiers in the cascade. The variable resistor can be optionally coupled to a feedback circuit which uses sensed bias current to maintain a desired bias level over temperature.
The present invention enables two or more broadband amplifiers to be cascaded and fabricated in a single integrated circuit chip, thereby saving the expense of assembling the cascaded amplifiers in a hybrid microcircuit using a coupling capacitor. The cascaded amplifiers have improved frequency response performance due to inclusion of a distributed and adjustable level-shifting network.